Same reason you can't talk about flaws in people's favorite shows rationally. Nostalgia doesn't want to be contradicted. I'm personally glad they canceled the show when they did. It had become almost cartoonish in its plot. The plan for the next season was to have her join the FBI. We're supposed to believe that she helped Duncan sneak out of the country, under the nose of the FBI, in season 2, but they're going to accept her as an agent in season 4. The first thing you do when you join is take a polygraph. But I'm sure her super sleuth powers would plot crowbar their way around that too.